This invention relates to an operational amplifier circuit operative over a range of common mode input voltages including substantially the amplifier positive and negative supply voltages.
The common mode input voltage range over which conventional bipolar transistor operational amplifiers operate is generally limited to common mode input voltages (the average of the input voltages to the amplifier) offset from the positive or negative voltage of the amplifier power supply. This input voltage limitation is due principally to the transistor emitter-base junctions between the input terminals and the positive or negative power supply terminal in the input bipolar transistor differential amplifier stage. For example, if the input differential amplifier stage is a PNP transistor differential amplifier, the PNP transistors are biased off when the input voltages are within the base-emitter junction voltage of the positive supply voltage so that the amplifier is unresponsive to the input voltages. Conversely, if the input differential amplifier stage is an NPN transistor differential amplifier, the NPN transistors are biased off when the input voltages are equal to or less than the base-emitter junction voltage (relative to the negative supply voltage) so that the amplifier is ineffective to compare the input voltages.
In some operational amplifier applications, it may be desirable to have the capability to respond to common mode input voltages that extend over a range that includes substantially the negative and positive amplifier supply voltages.